When the California Air Resources Board (ARB) adopted automotive exhaust emission standards for Super Ultra-Low-Emission Vehicles (SULEV), new challenges were encountered for accurately measuring exhaust emissions. This is especially true for measuring NMOG emissions (NMHC and carbonyls) where the SULEV standard is 0.010 g/mi. One of the challenges in accurately measuring NMHC emissions is to find a clean sample bag material that has no or very low outgassing of hydrocarbons. Tedlar, the bag material commonly used for exhaust emission sampling, has been found to emit N,N- dimethylacetamide (DMAc), which interferes with hydrocarbon measurements and can contribute to significant error in SULEV hydrocarbon emission measurements.
Several fluorocarbon materials were tested for hydrocarbon (HC) outgassing and carbon dioxide (CO2) permeation. The materials include Tedlar, Baked Tedlar, KynarFlex 2750, Baked KynarFlex 2800, Teflon FEP, TFM TFE, Tefzel, and Halar. The test methods included measuring HC outgassing of sample bags filled in dry zero air at 60 minutes and measuring carbon dioxide permeation of sample bags filled with 2% CO2 at 60 minutes. Since moisture content of the source gas may affect outgassing, tests were also performed using humidified zero air. Based on the potential effect of outgassing on hydrocarbon measurements and CO2 permeation on fuel economy measurements, material performance criteria are recommended. Tentative criteria are that hydrocarbon outgassing should be less than 15 ppbC1 at 60 minutes using both dry and humidified zero air as source gases, and CO2 permeation should be less than 0.2% at 60 minutes using a 2% CO2 source gas. Of the material tested, only baked Tedlar met both criteria. Some of the other materials tested may meet criteria using alternative treatments of the material.
Since a precise and accurate hydrocarbon gas analyzer is needed for this testing, this paper also describes the calibration and determination of the limit of detection (LOD) of the Horiba 726LE Flame Ionization Detector (FID) hydrocarbon analyzer in the Horiba MEXA LE emission analyzer system. The LOD was calculated to be 3 ppbC1. Such precision supports the reliability of the hydrocarbon outgassing data.